Mobile electronic devices are common in consumer, industrial and military environments. Due to their portable nature, mobile electronic devices typically rely on a portable electrical power source such as a battery.
A new type of portable electrical power source has arisen out of several technological breakthroughs, namely developments in micro-scale combustion (micro-combustion) and high-efficiency thermoelectric materials.
The advent of these two technologies enables electrical power generation using the high energy content of liquid hydrocarbon fuels such as propane, butane, kerosene, JP-8 or gasoline in such small form factors as to be compatible with mobile applications. Liquid hydrocarbon fuel has a very high energy density in the range of 70 to 100 times that of the current lithium-ion based batteries. Given this high energy content, even a modest energy conversion efficiency of 10% results in potentially a ten times improvement in current battery energy density.
What is currently lacking is a mobile electrical power system that combines the above technologies to accomplish miniature power generation with features such as a MEMS-based micro-combustion power system with multiple cells capable of providing sustained power levels of one to 50 watts. This relatively high power is an enabling technology advantageous for use in miniaturized smart munitions or to achieve greater autonomy and improved flight control in military systems.
Further needed is a micro-combustion power system that has a capacity in the range of 10 to 200 watts-hours. In this range, a micro-combustion power system exceeds the performance of electrochemistry batteries or fuel cells with a potential advantage of in the range of eight times higher energy density than lithium-ion.
Thermal and liquid reserve batteries generally separate the electrolyte from active electrodes and maintain the electrolyte in solid state until activation. Micro-combustion power systems have similar design advantages in that the fuel is physically separated from the energy converter chips. Until the fuel is channeled into the microcombustor and activated, no electro-chemical action takes place, thereby enhancing the reliability of the system.
One of the main drawbacks in the prior art is the power consumption in existing micro combustion power supplies relating to the use of air injector fans necessary to flow the fuel and air mixture through the microcombustor system. In practice, this requirement means the pressure drop in the entire flow path within the microcombustion power system must be kept to a minimum, starting from pumping the oxidant (air) into the pre-heating chamber, through the microcombustor combustion volume, the post-combustion volume and finally exhausted through the outlet port. The pressure drop through the heat exchanger components is relatively high because of the long length needed to achieve efficient convective heat transfer to the thermoelectric generator element.
The invention disclosed herein overcomes the above deficiencies in prior art micro-combustion power supply devices by providing is a dual path, counter-flow system. By dividing the microcombustor into two sections, the invention recovers exhaust heat by placing a post-combustion heat exchange structure downstream of each microcombustor to pre-heat the opposing cold air/fuel mixture stream.
The resulting benefit is an air/fuel mixture flow arrangement with two direct opposing flow paths and a minimum pressure drop along each of the paths.
The above invention is desirably implemented as a MEMS-based micro-combustion power system comprising micro-machined silicon structures that are small and lightweight and can be easily packaged to protect the device from harsh operating environments.